This invention relates to arrangements for holding magnets on components such as rotors of electrical machines that rotate at high speed.
In conventional permanent magnet electrical machines, magnets are radially retained on a rotor by nonmagnetic caps which are keyed into adjacent pole pieces or by providing angled sides on the magnet which are radially retained by wedged-shaped adjacent laminated pole pieces. Such arrangements, however, do not provide sufficient strength to hold a magnet securely in position when subjected to the radial accelerations encountered in high speed machines. In such arrangements, moreover, the pole pieces are subjected to increased stress as a result of the added radial load of the magnet during high speed operation. Furthermore, when angled sides of a magnet are engaged by wedged-shaped adjacent pole pieces, the radial location of the magnet may change, causing imbalance in the rotor. Also, such magnet support arrangements introduce flux path discontinuities in dovetails and oblique surfaces of the components in the magnetic flux path which decrease the efficiency of the machine. Moreover, the use of mechanical fastening arrangements such as bolts or screws requires access holes through magnet holders or pole pieces to permit insertion of fastening devices.
The Iwata U.S. Pat. No. 5,811,908 discloses U-shaped permanent magnet retention channels in which the outer ends of the channel walls have projections to be received in corresponding grooves in adjacent pole pieces and the entire channel is made of a single piece of magnetic material.
In the patent to Irie et al. U.S. Pat. No. 5,973,435 permanent magnets are assembled within protective nonmagnetic holders having nonmagnetic metal facings with projections by which they are secured to a nonmagnetic connecting band.
The Kloosterhouse et al. U.S. Pat. No. 5,191,255 shows a permanent magnet mounted in a U-shaped channel having laterally projecting tabs which receive screws for affixing the channel to a rotor.
The Burson U.S. Pat. No. 4,179,634 discloses a magneto rotor having permanent magnets which are retained in a cavity formed in the rotor by pole pieces disposed on opposite sides of the magnet and retained by retaining pins received in axially extending slots in the pole pieces.
The Patent to Morill U.S. Pat. No. 2,516,901 discloses a permanent magnet rotor having a hub to which permanent magnets are affixed by screws engaging shoulders on the magnets. In addition, rings may be cast around the shoulders of the magnets and the screws to assist in retaining the magnets on the hub.
In the magneto shown in the Iwata et al. U.S. Pat. No. 5,811,908, U-shaped permanent magnet retention channels made of magnetic material have projections at the outer ends of the walls which are received in corresponding grooves in adjacent pole pieces.
According to the Tomite et al. U.S. Pat. No. 4,745,319, permanent magnets are secured to the inner surface of a surrounding yoke by attaching one surface of a U-shaped elastic retainer member to the inner surface of the yoke and attaching the ends of the U-shaped retainer to edges of adjacent magnets which are to be retained in the yoke.
The Lechner et al. U.S. Pat. No. 6,150,746 discloses a U-shaped permanent magnet retaining channel made of nonmagnetic material having tabs that are welded to pole plates.
Accordingly, it is an object of the present invention to provide a magnet retention arrangement for high speed electric machine rotors which overcomes disadvantages of the prior art.
Another object of the invention is to provide a magnet retention arrangement for high speed rotors by which a magnet is securely retained in the radial direction in a rotor operating at high speed without introducing flux path discontinuities.
These and other objects of the invention are attained by providing a magnet retainer which is a U-shaped channel member made of nonmagnetic material which is supported from a rotor hub by interlocking connections between the radially inner ends of the sides of the channel member and the hub. Preferably, the inner ends of the sides of the channel member have lateral projections which are received in corresponding grooves in the rotor hub. The magnets may be inserted into the magnet retention channel before or after the channel is slidably inserted in the corresponding grooves in the nonmagnetic core. By providing interlocking engagement between the hub and the channel member, the necessity for insertion of mechanical fasteners such as bolts or screws is avoided. Moreover, by using a channel member which is made of nonmagnetic material the necessity for bonding or forming a channel member from different parts which are magnetic and nonmagnetic, and the resultant reduction in strength in the channel member, is avoided.